In the gas phase process for production of polyolefins such as polyethylene, a gaseous alkene (e.g., ethylene), hydrogen, co-monomer and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler (heat exchanger). The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by the fluidizing gas which is passed through a distributor plate near the bottom of the reactor vessel. Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in the external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feedstreams are added to maintain the desired reactant concentrations.
Products generally have similar properties based on the process conditions and additives to the process system. One such property of a polyolefin is shear thinning. Shear thinning can be defined as a property of substances where the viscosity of the substance decreases in response to an increasing rate of shear to the substance. Melt flow ratio (MFR) is a ratio of two different melt flow index measurements, and is used to quantify the shear-thinning of the polymer. As is well known, melt flow index measurements measure the rate of extrusion of thermoplastics through an orifice at a prescribed temperature and load, and are often used as a means to discern molecular weight of the overall polymer.
Typically, to create products with different properties from a reactor, very different additives and/or process conditions must be used. For example, temperatures, pressures, humidity, etc., may be controlled and tuned to different levels to create a particularly desired product.
However, altering the process conditions, sometimes only by a small percentage, can have a very negative effect on the efficiency of the process, on the yield from the process, on product properties, and may even ruin the desired product rendering the process ineffective. Accordingly, it is generally preferred to operate continuously under an unchanging set of operation conditions.
What is therefore needed is a way to produce polyolefins with different properties, such as shear thinning, without incurring great cost and/or downtime to the reactor.